1 Project Summary 2 3 Neuromodulation therapy using vagal nerve stimulation (VNS) is approved for drug resistant epilepsy and 4 depression. Clinical protocols selected for VNS therapy appear empirical with few links to scientific principle but 5 recommend use of the highest current intensities that patients can tolerate. When applied to the vagus nerve 6 trunk this regime of ?dosing? intensity shocks may not result in the greatest efficacy as might be expected with 7 maximal drug dosing. In clinical trials directed at drug resistant heart failure patients, threshold bradycardic 8 intensity (BI) is recommended with similar limiting side effects. Our global hypothesis is that VNS activates 9 myelinated vagal afferents to drive central nervous system (CNS) pathways that contribute to beneficial 10 therapeutic outcomes. In this proposal, our aims focus on weak-intensity, vagal afferent activation of the central 11 nervous system. In our Preliminary Results, BI in rats activated only myelinated vagal afferents and triggered 12 monosynaptic action potentials in a small proportion of neurons in the nucleus of the solitary tract (NTS). 13 Surprisingly, these studies indicate that these same VNS stimuli activated an extensive network of NTS neurons 14 only indirectly and included 2nd order neurons only directly connected to unmyelinated vagal afferents as well as 15 higher order neurons with no direct vagal afferent contacts. Thus, VNS activates most NTS neurons indirectly 16 and this spontaneous activation represents an effective spread or amplification of an initially minor activation to 17 extend to a much wider group of NTS neurons by unknown mechanisms. We propose Specific Aims supported 18 by Preliminary Results that will identify the mechanisms for this amplification and spread of excitation. We 19 combine in vivo as well as in vitro NTS recordings. Aim 1 will focus on potential mechanisms within NTS while 20 Aim 2 assesses contributions of neurons outside of NTS. Aim 2 proposes that VNS activation of supramedullary 21 neurons within the paraventricular nucleus of the hypothalamus (PVN) that activate NTS neurons producing 22 unsynced action potentials. Aim 3 evaluates potential changes in these vagal-NTS neuron interactions induced 23 by therapeutic VNS protocols. Throughout, focus will be on formulating optimized activation protocols of low 24 intensity, myelinated selective afferent activation through use of frequency and patterning (e.g. bursts) to 25 facilitate increased NTS and NTS associated pathway activation. Aim 4 evaluates the therapeutic efficacy of 26 VNS to relieve heart failure in a rat model. This proposal directly addresses a fundamental knowledge gap of 27 neuromodulation. Success in answering these Aims is the first step to optimization of VNS for therapeutic 28 efficacy. Greater knowledge of visceral afferent processing should drive new stimulation approaches in other 29 clinical applications. We aim to provide a knowledge foundation necessary for deliberate, evidence-based criteria 30 for VNS therapy to improve clinical outcomes. 31